Emulsions may be broadly defined as metastable colloidal dispersions of liquid droplets in another liquid phase. Typically, emulsions are disperse systems comprising at least two liquids which are virtually insoluble in one another. Besides the said at least two liquid phase components and optionally solid particles and/or one or more emulsifiers, the formation of an emulsion requires energy and occurs at conditions far from equilibrium. In general, the formation of an emulsion comprises the two following steps:                in an initial step, at least two components are pre-mixed, the said at least two components being originally liquid phase immiscible, the pre-mixing preferably being in the presence of a suitable amount of one or more emulsifiers, in order to create droplets of a dispersed liquid phase in another continuous liquid phase;        thereafter, the droplets resulting from the initial step are disrupted by shear forces or by local pressure differences, i.e. by inertial forces, thus resulting in a more stable emulsion of usually smaller droplets.        
At present, different types of mechanical emulsification processes are used in the production of finely dispersed emulsions, each process requiring specific equipment. Within these emulsification systems, four major categories may be recognized:                droplet disruption in a high shear rotor-stator system,        droplet disruption by ultrasound,        droplet disruption in high-pressure systems, and        droplet formation at micropores (using microporous membranes or microchannels).Also non-mechanical processes may be applied, such as precipitation of the dispersed phase previously dissolved in the continuous phase, phase inversion method and phase inversion temperature method.        
Emulsions may either be produced directly as consumer products or as intermediates for use within a broad range of industrial applications including, in a far from exclusive list, food, paints, cosmetics, pharmaceuticals, explosives, rocket fuel, lubricants, foam-controlling agents, etc. Most industrial applications or consumer products require that emulsions have maximal storage stability. The storage stability refers to the period of time during which the emulsion can be kept before it separates again into different phases. Mechanisms that can be identified in the process of breaking down an emulsion include the so-called Ostwald ripening, creaming, aggregation, coalescence, and partial coalescence. The process of breaking down an emulsion can be influenced or monitored, and therefore storage stability can be controlled or increased, in the two following ways: using mechanical devices to control the size of the dispersed droplets and/or adding stabilising chemical additives or emulsifiers in order to keep the emulsion dispersed.
Emulsions have great importance in the plastics (i.e. polymers) industry, especially in the detergents and cleaning products industries, in the production of lubricants, cosmetic, veterinary or pharmaceutical compositions (e.g. creams and ointments) and, in particular, in food products technology as well. Since many emulsions comprise at least one hydrophilic liquid and at least one lipophilic liquid, a further distinction is usually made, depending on the nature of the internal, disperse phase, between oil-in-water emulsions and water-in-oil emulsions. The internal or the external phase of the emulsion may itself in turn be a disperse system and may, for example, include particles of solids dispersed in the respective liquid phase, a system of this kind being also referred to as a multiphase emulsion. Owing to the interfacial tension which exists between the droplets of the internal phase and the droplets of the continuous, external phase, emulsions are in general thermodynamically unstable and thus after some time a phase separation occurs which may be induced, for example, by droplet sedimentation or coagulation. In order to prevent such phase separation it is common, during emulsion manufacturing, to add emulsifying auxiliaries, such as emulsifiers (which lower the interfacial tension) or stabilisers (which, for instance, prevent or at least greatly retard the sedimentation of the droplets, by increasing the viscosity of the continuous, external phase).
When the at least two liquid phase components of an emulsion are mixed together, the initial result is a coarsely disperse crude emulsion. By supplying mechanical energy, the large drops of the crude emulsion are broken up and the desired fine emulsion is formed. The smallest droplet size achievable in the last step of the emulsification process depends not only on the respective input of power in the emulsifying equipment but can be also critically influenced by the nature and concentration of the emulsifying auxiliaries. For example, in order to produce ultra-thin emulsions, it is essential that the new interfaces which are formed mechanically be occupied very rapidly by the emulsifier in order to prevent coalescence of the droplets.
The average size of the droplets of the disperse phase can be determined in accordance with the principle of quasi-elastic dynamic light scattering, for instance by using a Coulter N4+ particle analyser commercially available from Coulter Scientific Instruments.
A wide variety of liquid dispersing machines are used for producing emulsions. Emulsions of medium to high viscosity are produced mainly by means of rotor-stator systems, such as colloid mills or gear-rim dispersing machines. Low-viscosity emulsions have to date been produced mainly using high-pressure homogenizers, in which case the crude emulsion under a pressure of between about 100 bars and 1,000 bars is discharged through the about 10 to 200 μm high radial gap of a homogenizing nozzle. It is assumed that drop break-up in this case is mainly attributable to the effect of cavitation. One specific design of a high-pressure homogenizer is the microfluidizer, which operates at relatively low pressures of about 100 bars. However, high-pressure homogenizers are not without disadvantages. Especially when emulsifying polymerizable systems or when producing multiphase emulsions including solid particles, the narrow radial gap of the homogenizing nozzle may easily become clogged. The subsequently required cleaning is time-consuming and complex. Moreover, the high pressures used in this type of equipment entail sealing problems, especially when using liquid components which attack the equipment sealants. A further disadvantage of high-pressure homogenizers is that drop size and throughput are closely linked with each other. Such equipment is therefore unsuitable for producing emulsions in whose disperse phase it is intended to disperse solid particles.
Liposomes may be defined as vesicles in which an aqueous volume is entirely enclosed by a bilayer membrane composed of lipid molecules. When dispersing these lipids in aqueous media, a population of liposomes with sizes ranging from about 15 nm to about 1 μm may be formed. The three major types of lipids, i.e. phospholipids, cholesterol and glycolipids, are amphipathic molecules which, when surrounded on all sides by an aqueous environment, tend to arrange in such a way that the hydrophobic “tail” regions orient toward the center of the vesicle while the hydrophilic “head” regions are exposed to the aqueous phase. According to this mechanism liposomes thus usually form bilayers.
Several types of liposomes are known in the art. Referring to their physical structure, the more easily accessible type of liposomes consists of multilamellar vesicles (hereinafter referred to as MLV, according to standard practice in the art), i.e. onion-like structures characterized by multiple membrane bilayers, each separated from the next by an aqueous layer, usually having a size between about 100 nm and 1 μm, which known e.g. from U.S. Pat. No. 4,522,803 and U.S. Pat. No. 4,558,579. Their production can be reproducibly scaled-up to large volumes and they are mechanically stable upon storage for long periods of time.
By contrast, small unilamellar vesicles (hereinafter referred to as SUV, according to standard practice in the art) usually having a size between about 15 nm and 200 nm, possess a single bilayer membrane and are usually difficult to prepare on a large scale because of the high energy input required for their production and of the risks of oxidation and hydrolysis. In addition, SUV are thermodynamically unstable and are susceptible to aggregation and fusion. Furthermore, as the curvature of the membrane increases in SUV, it develops a degree of asymmetry, i.e. the restriction in packing geometry dictates that significantly more than 50% and up to 70% of the lipids making up the bilayer are located on the outside. Because of this asymmetry, the behaviour of SUV is markedly different from that of bilayer membranes comprising MLV or from that of large unilamellar vesicles (the latter, hereinafter referred to as LUV, usually having a size between about 100 nm and 1 μm).
Referring to their chemical structure, liposomes may be made from neutral phospholipids, negatively-charged (acidic) phospholipids, sterols and other non-structural lipophilic compounds. For instance, a population of detergent-free liposomes having a substantially monomodal distribution (i.e. unilamellar vesicles) about a mean diameter greater than 50 nm and exhibiting less than a two-fold variation in size may be produced (e.g. according to EP-B-185,756) by first preparing multilamellar liposomes and then repeatedly passing them under pressure through a filter having a pore size not more than 100 nm. For a detailed description of liposomes and methods of manufacturing them, reference is hereby made to Liposomes, a practical approach (1990), Oxford University Press. Liposome manufacturing and quality control encounters many of the difficulties set forth herein-above in respect of other dispersed systems including multiphase emulsions.
Thus there is a need in the art for providing an economical and improved alternative to the existing mechanical and non-mechanical processes for making emulsions. In particular, there is a need in the art for providing an emulsification process which avoids the requirement of complex mechanical equipment and the associated maintenance costs, while resulting in a desirable droplet size within a limited period of time and simplifying its quality control procedure, and while achieving a prolonged storage stability of the resulting emulsion. There is also a need in the art for modulating the characteristics, such as micelle size, of an emulsion either during or after its manufacturing. All the above needs apply to liposome manufacturing as well, more specifically including improving trapping efficiency and stability of multilamellar liposomes.